1. Field of the Invention
This invention relates to linear light-emitting diode (LED) lamps and more particularly to a linear LED lamp free of shock hazard.
2. Description of the Related Art
Solid-state lighting (SSL) using light-emitting diodes (LEDs) has received much attention in general lighting applications today. It is expected that such SSL will be a mainstream in the near future because of its potential for more energy savings, better environmental protection (no hazardous materials used), higher efficiency, smaller size, and much longer lifetime than conventional incandescent bulbs and fluorescent tubes. As LED technologies develop with the drive for higher energy efficiency and cleaner technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock become much more important and need to be well addressed.
In retrofit application of a linear LED (LL) lamp to replace an existing fluorescent tube, one must remove the starter or ballast because the LL lamp does not need a high voltage to ionize the gases inside the gas-filled fluorescent tube before sustaining continuous lighting. LL lamps operating at AC mains, such as 110, 220, or 277 VAC, have one construction issue related to product safety, which needs to be resolved prior to wide acceptance. This kind of LL lamps always fails a safety test, which measures through lamp leakage current. Because the line and the neutral of the AC main apply to both opposite ends of the tube lamp when connected, the measurement of current leakage from one end to the other consistently results in a substantial current flow, which may present a risk of shock during relamping.
LEDs have a long operating life of 50,000 hours, much longer than conventional lighting devices do. One of the most important factors that detrimentally affect the operating life of an LED-based lamp is high junction temperature of LEDs. While LEDs can operate 50,000 hours, the LED lamps do need a good thermal management in their heat sink design. A more efficient heat sink can effectively maintain the LED junction temperature at a lower value and thus prolong the operating life of LEDs. Conventionally, the most cost-effective heat sink is made of metal. One of the drawbacks of using a metal as a heat sink in LL lamp application is electrical conductivity because a shock hazard may occur when consumers touch the heat sink that is not well insulated from the LED printed circuit board (PCB) and the internal driver that powers the LEDs.
Today, such LL lamps are mostly used in a ceiling light fixture with a power switch on the wall. The ceiling light fixture could be an existing one used with fluorescent tubes but retrofitted for LL lamps or a specific LL lamp fixture. The drivers that provide a proper voltage and current to LEDs could be internal or external ones. An LL lamp with an external driver uses a remote driver that provides a low DC voltage to the lamp and is thus inherently electric-shock free if the driver meets the dielectric withstand standard used in the industry. On the other hand, LL lamps with an internal driver and a metallic heat sink present the shock hazard during relamping or maintenance, as mentioned in the previous paragraph, when a substantial leakage current flows from any one of AC voltage input through the metallic heat sink to the earth ground. Despite this disadvantage, LL lamps with an internal driver and a metallic heat sink still receive acceptance because they provide a long life, a stand-alone functionality, and an easy retrofit for an LL lamp fixture.
Any LL lamps will more or less produce a small amount of leakage current through an internal electrical contact and the metallic heat sink because of the voltages applied and internal capacitance present in the lamps. When there exist design flaws, such as un-isolated design of an LED driver, or material and workmanship defects, the electrical insulation in the LL lamp can break down, resulting in a substantial leakage current flow. It mostly happens for small gaps between current-carrying conductors and the earth ground. When an LL lamp is operated under normal conditions, environmental factors such as dirt, contaminants, humidity, vibration, and mechanical shock can weaken the insulation and facilitate the current to flow through these small gaps, thus creating a shock hazard to anyone who comes into contact with the metallic heat sink on the faulty LL lamps if care is not well taken.
As consumerism develops, consumer product safety becomes extremely important. Any products with electric shock hazards and risk of injuries or deaths are absolutely not acceptable to consumers. However, commercially available LL lamps with internal drivers and a metallic sink, which are used to replace fluorescent tubes, fail to provide a solution to these problems. In the present invention, an electrically insulating but thermally conductive heat sink with an efficient heat-dissipation structure in addition to two shock protection switches used on the lamp bases is adopted to fully protect consumers from possible electric shock injuries and deaths during relamping or maintenance.
Referring to FIG. 1 and FIG. 2, a conventional LL lamp 100 without shock protection switch comprises a metallic housing 110, which also serves as a heat sink, with a length much greater than its radius, two end caps 120 and 130 each with a bi-pin 180 and 190 (not shown in FIG. 1) respectively on two opposite ends of the metallic housing 110, LED arrays 140 with a plurality of LEDs 170 on an LED PCB 150, and an LED driver 160 used to generate a proper DC voltage from the energy supply of the AC main through internal electrical wires 151 and 152 and to provide a proper current to supply the LED arrays 140 through an internal wire connection 161 and 162 such that the LEDs 170 on the PCB 150 can emit light. The LED PCB 150 is glued on a surface of metallic housing 110 by an adhesive with its normal parallel to the illumination direction. The bi-pins 180 and 190 on the two end caps 120 and 130 are electrically connected to an AC main, either 110 V, 220 V, or 277 VAC, through two electrical lamp sockets (not shown) located lengthways in an existing fluorescent tube fixture (not shown). The two lamp sockets in the fixture are electrically connected to the line (L) and the neutral (N) wire of the AC main, respectively.
To replace a fluorescent tube with an LL lamp 100, one inserts the bi-pin 180 at one end of the LL lamp 100 into one of the two lamp sockets in the fixture and then inserts the bi-pin 190 at the other end of the LL lamp 100 into the other lamp socket in the fixture. When the line of the AC main applies to the bi-pin 180 through a lamp socket, there exists a shock hazard as long as the bi-pin 190 at the other end is not in the lamp socket because consumers who replace the linear LED lamp may touch the exposed bi-pin 190. The excessive current will flow from the bi-pin 180, the internal wire 151, the driver 160, the internal wire 152, and the bi-pin 190 to earth through his or her body—a shock hazard. This is most likely to happen in practice. To prevent consumers from injury for this shock hazard, Underwriters Laboratories (UL), uses its standard, UL 935, Risk of Shock During Relamping (Through Lamp), to do the current leakage test and to determine if LL lamps under test meet the consumer safety requirement.
On the other hand, when the line or neutral wire of the AC main is connected to the bi-pin 180 through a lamp socket, no matter whether the bi-pin 190 at the other end is in the lamp socket or not, there exists another shock hazard because at this time, if a high voltage from a lighting strike, for example, applies to the AC main of the LL lamp, which happens to be a faulty one as mentioned above, a high voltage breakdown, from the insulation-weakest point along an electrical path from the bi-pin 180, through the internal wires 151, 161, and 162, the LED driver 160, and LED arrays 140 on the LED PCB, to the heat sink 110, will lead to an excessive leakage current flow to the heat sink 110. At this time, if the person who replaces the LL lamp 100 touches the housing 110, he or she will get electric shock because the current flows to earth through his or her body. This is also likely to happen in practice. To prevent consumers from injury due to this shock hazard, UL uses one of the procedures in UL 1993 Standards, Dielectric Voltage-Withstand Test, to determine if LL lamps under test meet the consumer safety requirements.
When an LL lamp is used as a lighting source, consumers used to use a power switch on the wall to turn the LL lamp power on or off. Intuitively, they just turn the LL lamp power off during relamping or maintenance and presume that it is safe without any shock hazards. But in practice, if the wiring is such that the neutral wire goes to the switch while the hot wire is connected all the time to the LL lamp fixture, then there exists shock hazards during relamping and maintenance because the consumers may touch the exposed bi-pin when the other bi-pin is still in the electric lamp socket. One of the solutions is to use two shock protection switches, one each on the two ends, such that the leakage current is blocked when either one of bi-pins is out of the lamp socket.
On the other hand, if the housing of an LL lamp is metallic, then there exists another shock hazard to the person who replaces the LL lamp and touches the housing if high voltage spikes occur. One solution is to use a facility switch on the LL lamp to manually turn off the internal electrical connections prior to replacement or maintenance. But in practice, the installation instruction in which the safety procedures are described may not be followed by a consumer, and thus the shock hazard is still a safety issue. It is, therefore, a responsibility of a lighting manufacturer to provide such an LL lamp with 100% safety guaranteed by design. In this case, the solution is to adopt an electrically insulating but thermally conductive heat sink, preferably made of amorphous plastic, such as TARFLON® Polycarbonate G131Z1, or thermoplastic, such as Stanyl® TC 153 and TC 501, to replace the metallic one.
Plastic-based materials are widely used in many applications from consumer products to aerospace structures. But such materials have never been used LL lamps as a heat sink. A basic plastic material is either not rigid enough to support a fine honeycomb structure in long length applications such as 2-, 4-, and 5-foot LL lamps, or its thermal conductivity, 0.04˜0.5 W/m° K, not high enough to efficiently conduct the heat generated by operating LEDs to the outer surface, resulting in a shortened LED life. On the contrary, the amorphous plastic or thermoplastic used in the LL lamps according to this invention provide much higher mechanical strength and thermal conductivities than pure plastics do. However, such a plastic heat sink with a simple tube shape alone still cannot dissipate heat as efficiently as aluminum heat sink widely used in the LL lamp applications. Therefore, one needs to use a more complex heat sink structure to improve heat dissipation.
Fortunately, current technology of injection molding can be used cost-effectively with the thermally conductive plastics to build not only a simple toothed but a complex honeycomb structure. Nevertheless, it can only produce an object of a length about 20 centimeters. Ultrasonic welding is thus needed for joining such plastic sections to achieve the desirable lengths for the heat sink. The process by which high-frequency vibration energy is directed to the interfaces to be joined is rapid and can be automated. The welding automation that integrates several modules used in the LL lamps also makes mass production and low cost possible, which helps such solid-state devices find faster adoption in general lighting applications to save energy and protect environments.